Investigations of Co(III) porphyrin complexes have in part been motivated by their use as models for cobalamin species such as coenzyme B12, and in part by their reactivity with nucleic acids. Interactions of Co(III) porphyrins with biomolecules are governed by their axial ligations and no Co(III) porphyrin complex with a single strong field, s-donating axial ligand, such as nitrogen, has been reported. Unlike Co(III) porphyrins, pentacoordinate Fe(III) porhyrins are found in many hemoproteins and have also been synthesized. For example, the met (oxidized, ferric) form of the is64 to Leu mutant of the oxygen-carrying protein myoglobin [Mb(H64L)] contains a pentacoordinate metal with an axial histidine ligand, whereas in the native protein, the second axial ligand is a water molecule. No water molecule was found near the metal in the mutant. In this study, Co(III) protoporphyrin-substituted Mb and Mb(H64L) are studied by x-ray absorption spectroscopy. The absorption edge energy for both proteins occur near 7729 eV, suggesting that both Hare hexacoordinate. The sixth ligand of Co(III)-substituted Mb has Hbeen suggested to be His64, rather than water as in the Fe(III) protein. Since neither His64 nor water (presumably) are present in the Co(III)-substituted mutant, the current results are surprising. Further studies are being conducted to identify the sixth ligand in Co(III) Mb(H64L).